Method and system for acquiring ultra-wide-bandwidth communications signals using sequential block searches

ABSTRACT

A method and system acquire a received impulse radio signal by first searching, with respect to time, a region of the impulse radio signal using a first template signal to locate a block including a signal cell, and second searching, with respect to time, the block using a second template signal to locate the signal cell to acquire the received impulse radio signal.

CROSS REFERENCE TO RELATED APPLICATION

[0001] A claim of priority is made to U.S. provisional applicationserial No. 60/451,442, “Method and Apparatus for Rapid SignalAcquisition in Ultra Wideband Communications Systems,” filed Mar. 3,2003.

FIELD OF THE INVENTION

[0002] The invention relates generally to impulse radio signals, andmore particularly to acquiring ultra-wide-bandwidth signals.

BACKGROUND OF THE INVENTION

[0003] Before a received ultra-wide-bandwidth (UWB) signal can bedemodulated, a template signal must be aligned with the received signal.The purpose of the alignment is to determine a relative delay of thereceived signal with respect to the template signal. This process iscalled signal acquisition.

[0004] Conventionally, the alignment is performed by a serial search ofpossible delay times of cells in an uncertainty region, see Simon etal., “Spread Spectrum Communications Handbook,” McGraw-Hill, New York,1994. Each different search location, i.e., time interval, with respectto time, is called a cell. If the signal exists at a delay location,then that cell is called a signal cell. In that method, the receivedsignal is correlated with the template signal, and an output of thecorrelation is compared to a threshold. If the output is lower than thethreshold, then the template signal is shifted by some amount time. Theshifted amount time corresponds usually to a resolvable path interval.This information is then used to repeat the correlation until the outputexceeds the threshold.

[0005] If the output of the correlation comes from a case where thesignal path and the template signal are aligned, it is called a signalcell output, otherwise, it is called a non-signal cell output. A falsealarm occurs when a non-signal cell output exceeds the threshold. Inthat case, time t_(p) elapses until the search recovers. This time iscalled the penalty time for false alarm.

[0006] Due to the short time resolution of UWB signals, seriallysearching all delay locations can take a long time. Therefore, thealignment method must be fast so that the time allocated for theacquisition of the UWB signal is reduced.

[0007] The mean acquisition time of a serial search is directlyproportional to the number of cells in the uncertainty region, N, forlarge N values, see Polydoros et al., “A unified approach to serialsearch spread-spectrum code acquisition-Part 1: General Theory,” IEEETrans. Comm., Vol. COM-32, pp. 542-549, May 1984. In a general form,FIG. 3 shows the basic operation of the prior art serial cell search ofan uncertainty region 300 with a template 301.

[0008] Therefore, there is a need for a method and system that canacquire UWB signals in a shorter time than the known prior art serialsearch techniques.

SUMMARY OF THE INVENTION

[0009] In impulse radio communication systems, such asultra-wide-bandwidth (UWB) communications, the received radio signalmust be acquired before the signal can be demodulated. Due to the veryshort time resolution of the narrow pulses of the UWB signal, aconsiderable number of possible signal locations, i.e., delays, must besearched in order to acquire the received signal. For this reason, fastsignal acquisition is very important for impulse radio communications.

[0010] Using a first template signal, a sequential block search method(SBS) according to the invention first determines a small region, withrespect to time, where the received signal cells are likely to exist.Then, this region is searched in more detail to find the exact delay ofthe signal using a second template signal.

[0011] The correlation of the received signal with the first templatesignal effectively adds some cell outputs. The result is then used as acriterion to determine if that region, generally, contains signal cells.

[0012] If the correlated output for the region exceeds a specifiedthreshold value, then the block is searched serially in detail using thesecond template signal.

[0013] The correlation intervals for the sequential block search and theserial cell search steps do not have to be the same. In fact, thecorrelation interval of the block search is usually chosen to besubstantially longer in order to be able find the correct block with ahigh probability.

[0014] In harsh non-line-of-sight (NLOS) conditions, there are manymultipaths, which make signal acquisition more challenging. If the firstpath, or first few paths, need to be acquired in such conditions, thenan average block search method according to the invention can be used.In this method, a number of serial search outputs are averaged, and theamount of increase between successive average values is compared to athreshold to detect an edge of the start of the received signal.

[0015] If there is a significant increase in the average value, then theoutputs of the cells causing that increase are further searched todetermine the first path of the received signal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a block diagram of a sequential block search method andsystem according to the invention; and

[0017]FIG. 2 is a block diagram of an average block search method andsystem according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0018] Signal Model

[0019] In a binary phase-shift keyed random time hopping impulse radio(TH-IR) system, the transmitted signal can be represented by thefollowing model: $\begin{matrix}{{{S_{t\quad r}(t)} = {\sum\limits_{j = {- \infty}}^{\infty}\quad {d_{j}^{k}b_{\lfloor{j/N_{f}}\rfloor}^{k}{w_{t\quad r}( {t - {j\quad T_{f}} - {c_{j}^{k}T_{c}}} )}}}},} & (1)\end{matrix}$

[0020] where w_(tr) is the transmitted unit-energy pulse, T_(f) is theaverage pulse repetition time, N_(f) is the number of pulsesrepresenting one information symbol, and b is the information symboltransmitted, i.e., zero or one.

[0021] In order to allow the channel to be exploited by many users andavoid catastrophic collisions, a pseudo-random sequence {c_(j)} isassigned to each user. This sequence is called the time hopping (TH)sequence. The TH sequence provides an additional time shift ofc_(j)T_(c) seconds to the j^(th) pulse of the signal, where T_(c) issometimes called the chip interval. To prevent pulses from overlapping,the chip interval is selected to satisfy T_(c)≦T_(f)/N_(c).

[0022] We consider coded IR systems where the d_(j)'s are binary randomvariables, and where d_(i) and d_(j) are independent for i≠j, takingeach values ±1 with a probability of ½, see Fishler et al., “On thetradeoff between two types of processing gain,” 40^(th) Annual AllertonConference on Communication, Control, and Computing, 2002. This systemscan be regarded as an random-code division multiple access radio signal(RCDMA) system with T_(f)=T_(c). In this case, N_(f) represents theprocessing gain.

[0023] We define a sequence {s_(j)} as follows $\begin{matrix}{s_{j} = \{ {\begin{matrix}d_{\lfloor{j/N_{c}}\rfloor} & {{j - {N_{f}\lfloor {j\quad N_{c}} \rfloor}} = c_{\lfloor{j/N_{c}}\rfloor}} \\0 & {otherwise}\end{matrix}.} } & (2)\end{matrix}$

[0024] Then, assuming T_(f)/T_(c)=N_(c), without loss of generality,Equation (1) can be expressed $\begin{matrix}{{S_{t\quad r}(t)} = {\sum\limits_{j = {- \infty}}^{\infty}\quad {s_{j}b_{\lfloor{{j/N_{f}}N_{c}}\rfloor}^{k}{{w_{t\quad r}( {t - {j\quad T_{c}}} )}.}}}} & (3)\end{matrix}$

[0025] We assume that no data modulation is done during the acquisitionstage, that is b_(j) ^(k) _(/N) _(f) _(N) _(c) =1∀j. In this case, thereceived signal over a flat fading channel in a single user system canbe expressed as $\begin{matrix}{{{r(t)} = {{\sum\limits_{j = {- \infty}}^{\infty}\quad {s_{j}{w_{rec}( {t - {j\quad T_{c}} - \tau} )}}} + {\sigma_{n}{n(t)}}}},} & (4)\end{matrix}$

[0026] where w_(rec)(t) is the received UWB pulse, and n(t) is whiteGaussian noise with unit power spectral density. This modelapproximately represents the line-of-sight (LOS) case, with a strongfirst component.

[0027] The number of cells in an uncertainty region is taken to beN=N_(f)Nc. One of these cells is the signal cell, while the others arenon-signal cells.

[0028] Template Signal

[0029] Assuming no data modulation for the purposes of acquisition, thenthe template signal that is used in a serial search for the signal modelin Equation (3) can be expressed as follows: $\begin{matrix}{{{s_{m_{2}}^{(c)}(t)} = {\sum\limits_{n = {j\quad N_{c}}}^{{{({j + m_{2}})}N_{c}} - 1}\quad {s_{n}{w_{rec}( {t - {n\quad T_{c}}} )}}}},} & (5)\end{matrix}$

[0030] where m₂ is the number of pulses, over which the correlation istaken.

[0031] Sequential Block Search

[0032] For a sequential block search (SBS) according to the invention,there are two different template signals. The first template signal isused for searching a block of cells, while the second template signal issimilar to the one used in the serial search.

[0033] The first template signal for the signal model described inEquation (3) can be expressed as follows: $\begin{matrix}{{{s_{m_{1}}^{(b)}(t)} = {\sum\limits_{i = 0}^{K - 1}\quad {\sum\limits_{n = {j\quad N_{c}}}^{{{({j + m_{1}})}N_{c}} - 1}\quad {s_{n}{w_{rec}( {t - {n\quad T_{c}} - {i\quad T_{c}} - {( {b - 1} )K\quad T_{c}}} )}}}}},} & (6)\end{matrix}$

[0034] where B is the total number of blocks in the uncertainty region,each block including K cells, and where m_(l) is the number of pulses,over which the correlation is taken. For simplicity, it is assumed thatthe total number of uncertainty cells can be expressed as N=KB. Thevalue T_(c), is taken as the minimum resolvable path interval.

[0035] The output of the correlation of the received signal and thefirst template signal in Equation (6) is used as a quick test to checkif the whole block contains a signal cell, or not. The correlationoutput of the received signal and the second template signal is thenused in a detailed search of a block.

[0036] The index of the block that is currently being searched is b,with b=1 initially. Then, the SBS method can be described as follows:

[0037] 1) Check the b^(th) block using the first template signal s_(m) ₁^((b))(t).

[0038] 2) If the output of the b^(th) block is not higher than a blockthreshold, τ_(b), then, go to step 6.

[0039] 3) If the output of the b^(th) block is higher than the blockthreshold, τ_(b), then search the block in more detail, i.e.,cell-by-cell serial search with a signal threshold τ_(s), using thesecond template signal s_(m) ₂ ^((c))(t).

[0040] 4) If no signal cell is detected in the block, go to step 6.

[0041] 5) If the signal cell is detected in the block, DONE.

[0042] 6) Set b=(b mod B)+1 and go to step 1.

[0043] When a false alarm (FA) occurs in the serial search part, thesearch resumes with the next cell after C time units, which is thepenalty time in terms of frame time.

[0044] In step 5, “the signal cell is detected” means that the signalcell output exceeds the signal threshold, τ_(s). Similarly, in step 4,“no signal cell is detected” implies that the signal cell is not in theblock, or the output of the cell is lower than the signal thresholdτ_(s), even if the cell is in the block.

[0045]FIG. 1 shows the SBS method. The received signal 101 is correlated110 with the first template signal of Equation (6), and the output 111is compared 120 to the block threshold τ_(s).

[0046] If the block threshold is not exceeded 121, the decision unit hasa synchronization unit 130 adjusted 131 the delay of the first templatesignal, and another correlation 110 with the received signal isperformed.

[0047] When the block output 111 is higher than the block thresholdτ_(b), the second template signal in Equation (5) is employed and thecells in the block are serially searched. In other words, decision unit120 compares the outputs with the thresholds and decides if the signalis detected 122, or not 121, while the synchronization unit 130 adjusts131 the delays of the template signals and sends the corresponding oneto the correlation unit.

[0048] Average Block Search

[0049] An average block search method is appropriate in harsh NLOSconditions. The basic idea behind this method is to use an average valueof a number of serial correlation outputs in order to see a considerableincrease in the output values. This increase indicates the start of thesignal cells. The received signal in this case is expressed as:$\begin{matrix}{{{r(t)} = {{\sum\limits_{j = \infty}^{\infty}{\sum\limits_{l = 1}^{L}\quad {\alpha_{l}\quad s_{j}{w_{rec}( {t - {j\quad T_{c}} - \tau_{l}} )}}}} + {\sigma_{n}{n(t)}}}},} & (7)\end{matrix}$

[0050] where α₁, is the amplitude coefficient and τ_(l), is the delay ofthe l^(th) multipath component.

[0051] Consider the outputs of the correlations of the received signalwith the following template signal: $\begin{matrix}{{s_{m}^{(c)}(t)} = {\sum\limits_{n = {j\quad N_{c}}}^{{{({j + m})}N_{c}} - 1}\quad {s_{n}{{w_{rec}( {t - {n\quad T_{c}}} )}.}}}} & (8)\end{matrix}$

[0052] If the absolute values of the results of these correlations arez₁, . . . ,z_(N), then we can define $\begin{matrix}{{w_{i} = {\frac{1}{K}{\sum\limits_{j = {{i\quad K} + 1}}^{{({i + 1})}K}\quad z_{j}}}},} & (9)\end{matrix}$

[0053] assuming N=KB.

[0054] Let i be the index of the averaged block currently beingsearched, with i=0 initially. Then, the ABS method can be describedfollows:

[0055] 1) Check difference between successive averagesw_(i mod B)−w_((i−1)) mod B.

[0056] 2) If the difference is not higher than a first threshold τ_(a)go to step 6.

[0057] 3) If the difference is higher than τ_(a), checkz_((i mod B)K+1), . . . ,z_((i mod B)+1)K) serially, comparing to asecond threshold, τ_(c).

[0058] 4) If no signal cells detected, go to step 6.

[0059] 5) If signal cell(s) are detected, DONE.

[0060] 6) Set i=(i+1) mod B, and go to step 1.

[0061]FIG. 2 shows ABS method and system 200. In this embodimentmultiple correlators 210 averaging units 215 are used in parallel. Areceived signal r(t) 201 is first correlated 210 with a first templatesignals with different delays. Then, the absolute values of thesecorrelations are averaged 220 and compared to the previous averagedvalue by the decision unit 230. If there is a significant increase inthe average value and if any one of the serial search outputs in thecorresponding block exceeds the threshold, the signal is detected 231.

[0062] If no detection 232 occurs, then, the delays of the templatesignal are adjusted by the synchronization unit 240, and the same stepsare followed again.

[0063] Note that even though the block diagram is shown for the casewith K correlators and averaging units, the method and system can alsobe worked with only one correlator. In such a situation, the decisionunit can perform the averaging and comparison tasks by storing apredetermined number of outputs of the single correlator.

[0064] Effect of the Invention

[0065] The sequential block search method according to the inventionprovides a quick method to find the location(s) of a signal cell of aUWB signal. First, the method quickly determines a smaller region wheresignal cells are likely to exist. Then, it searches that region indetail to find the exact location of the signal. In this way, the timeto acquire the UWB signal can be reduced considerably. In fact, the meanacquisition time of the SBS method becomes proportional to the squareroot of N for large signal-to-noise ratios. In contrast, the meanacquisition time of a prior art serial search is directly proportionalto the number of cells in an uncertainty region. For practical values,the acquisition time using the SBS method is about the half of theserial search mean acquisition time.

[0066] In harsh multipath conditions, an average block search reducesthe acquisition time because the averaged values of serial searchoutputs are more reliable in detecting the start of the signal in someNLOS situations. In this way, instantaneous increases in the singleoutputs are smoothed so that the frequency of false alarms is reduced.It should be noted that the invention can also be used in directsequence-code division multiple access (DS-CDMA) systems.

[0067] Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications may be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

We claim:
 1. A method for acquiring a received impulse radio signal,comprising: searching, with respect to time, a region of the impulseradio signal using a first template signal to locate a block including asignal cell; and searching, with respect to time, the block using asecond template signal to locate the signal cell to acquire the receivedimpulse radio signal.
 2. The method of claim 1 wherein the impulse radiosignal is a random-code division multiple access radio signal.
 3. Themethod of claim 1 wherein the impulse radio signal is anultra-wide-bandwidth radio signal.
 4. The method of claim 1 wherein theimpulse radio signal is a direct sequence-code division multiple accessradio signal.
 5. The method of claim 1 wherein the impulse radio signalis a direct binary phase-shift keyed random time hopping radio signal.6. The method of claim further comprising: wherein a first correlationinterval associated with the first template signal is substantiallylonger than a second correlation interval associated with the secondtemplate signal.
 7. The method of claim 5 wherein the searching furthercomprises: time shifting the first and second template signals whilesearching; correlating the shifted first and second template signalsover the first and second correlation intervals to determine first andsecond outputs, respectively; and comparing the first and second outputsto first and second thresholds to locate the block and signal cell,respectively.
 8. The method of claim 1 wherein a transmitted impulseradio signal corresponding to the received impulse radio signal isrepresented by${{S_{t\quad r}(t)} = {\sum\limits_{j = {- \infty}}^{\infty}\quad {d_{j}^{\quad k}b_{\lfloor{j/N_{f}}\rfloor}^{k}{w_{t\quad r}( {t - {j\quad T_{f}} - {c_{j}^{k}T_{c}}} )}}}},$

where w_(tr) is a transmitted unit-energy pulse, T_(f) is an averagepulse repetition time, N_(f) is the number of pulses representing oneinformation symbol, and b is an information symbol transmitted, eitherzero or one, and a pseudo-random sequence {c_(j)} is assigned to eachuser of the impulse radio signal to provide an additional time shift ofc_(j)T_(c) seconds to the j^(th) pulse of the impulse radio signal,where T_(c) is a chip interval, and T_(c)≦T_(f)/N_(c), and where d_(j)are binary random variables, and where d_(i) and d_(j) are independentand taking each values of ±1 with a probability of ½, and a sequence{s_(j)} as defined follows by $s_{j} = \{ {\begin{matrix}d_{\lfloor{j/N_{c}}\rfloor} & {{j - {N_{f}\lfloor {jN}_{c} \rfloor}} = c_{\lfloor{j/N_{c}}\rfloor}} \\0 & {otherwise}\end{matrix},} $

to express the transmitted impulse radio signal${{as}\quad {S_{tr}(t)}} = {\sum\limits_{j = {- \infty}}^{\infty}\quad {s_{j}b_{\lfloor{{j/N_{f}}N_{c}}\rfloor}^{k}{w_{tr}( {t - {jT}_{c}} )}}}$

when T_(f)/T_(c)=N_(c), so that the received impulse radio signal isexpressed as${{r(t)} = {{\sum\limits_{j = {- \infty}}^{\infty}\quad {s_{j}{w_{rec}( {t - {jT}_{c} - \tau} )}}} + {\sigma_{n}{n(t)}}}},$

where w_(rec)(t) is a received pulse, and σ_(n)n(t) is white Gaussiannoise with unit power spectral density, and b_(j) ^(k) _(/N) _(f) _(N)_(c) =1∀j, and the number cells in the region is N=N_(f)N_(c), and T_(c)is a minimum resolvable path interval.
 9. The method of claim 8 whereinthe first template signal is expressed as${{s_{m_{1}}^{(b)}(t)} = {\sum\limits_{i = 0}^{K - 1}\quad {\sum\limits_{n = {jN}_{c}}^{{{({j + m_{1}})}N_{c}} - 1}\quad {s_{n}{w_{rec}( {t - {nT}_{c} - {iT}_{c} - {( {b - 1} ){KT}_{c}}} )}}}}},$

where m₁ is the number of pulses over a first correlation intervalassociated with the region, B is a total number of blocks in the region,each block including K cells, and the plurality of cells is N=KB. 10.The method of claim 8 wherein the second template signal is expressed as${{s_{m_{2}}^{(c)}(t)} = {\sum\limits_{n = {jN}_{c}}^{{{({j + m_{2}})}N_{c}} - 1}\quad {s_{n}{w_{rec}( {t - {nT}_{c}} )}}}},$

where M₂ is the number of pulses over a second correlation intervalassociated with the block.
 11. The method of claim 1 wherein thesearching of the region uses a block threshold, τ_(b), and the searchingof the block uses a signal threshold τ_(s).
 12. A system for acquiring areceived impulse radio signal, comprising: first means for searching,with respect to time, a region of the impulse radio signal using a firsttemplate signal to locate a block including a signal cell; and secondmeans for searching, with respect to time, the block using a secondtemplate signal to locate the signal cell to acquire the receivedimpulse radio signal.
 13. An apparatus for acquiring a received impulseradio signal, comprising: a correlation unit configured to search, withrespect to time, a region of the impulse radio signal using a firsttemplate signal to locate a block including a signal cell, and tosearch, with respect to time, the block using a second template signalto locate the signal cell to acquire the received impulse radio signal;a synchronization unit to time-shift the first and second templatesignals while searching; and a decision unit to compare first and secondoutputs of the correlation unit to first and second thresholds whilesearching the region and block respectively.